1. Field of the Invention
The invention relates to AZS (alumina-zirconia-silica) fused cast products having improved characteristics for use in a glass melting furnace. The invention relates more particularly to oxidized AZS products characterized by a homogeneous crystal structure and having elongate dendritic zirconia crystals in the active area thereof. The simultaneous presence of these characteristics provides these products with increased resistance to corrosion by molten glass.
2. Description of the Prior Art
Fused cast products (also referred to as xe2x80x9celectro-castxe2x80x9d products) are obtained by melting a mixture of appropriate raw materials in an electric arc furnace or by any other melting technique suited to the products concerned. The molten liquid is then cast in a mold to produce shaped components directly. The product is generally then subjected to a controlled cooling program to cool it to ambient temperature without it breaking. The skilled person refers to this operation as xe2x80x9cannealingxe2x80x9d.
AZS products have been known in the art for a number of decades and have supplanted products based only on alumina and silica. U.S. Pat. Nos. 2,271,366 and 2,438,552 describe the first improvements to AZS products. FR-A-1 208 577 teaches the production of AZS products under oxidizing conditions. The first generation products tended to release gas bubbles into the molten glass, leading to unacceptable defects in the glass. Changing to oxidizing production conditions improved the strength of the AZS products and the quality of the glass. Oxidized products are generally white-yellow to white-orange in color, whereas reduced products are white to white-gray in color.
AZS refractory products comprise different phases: alpha-alumina crystals (corundum), zirconia crystals and a vitreous phase. The alpha-alumina and the zirconia are partly combined in eutectic crystals.
The prior art provides sometimes contradictory teaching as to the nature and the shape of the crystals. U.S. Pat. No. 2,079,101 indicates that it is preferable to have a highly oriented crystal structure in which the crystals are parallel to each other and perpendicular to the faces of the cast block. FR-A-1 153 488 describes AZS products with an interleaved crystal structure which is advantageous in terms of improved corrosion resistance. The above products are first generation products, i.e. reduced products. However, the inventors of FR-A-1 153 488 disclose their invention only in relation to a very particular block shape and their microstructure analyses relate to only a small area of the block. They indicate that it is the chemical composition of the product that produces the required microstructures. In particular, they specify that the crystal structure of their invention is encountered only in a small area of the Al2O3-ZrO2-SiO2 system in which the silica content is from 16% to 20%. They also indicate that the presence of too high a proportion of sodium oxide has a harmful effect on corrosion resistance and that the Na2O/SiO2 ratio must be limited to 0.14. U.S. Pat. Nos. 4,791,077 and 5,171,491 indicate that there is a structural difference between the skin and the core of the components. They also teach that a structure with elongate and interleaved crystals is disadvantageous and propose a solution for obtaining components with a fine and uniform microstructure free from dendritic zirconia crystals.
The products commercially available at present are oxidized products, such as our ER-1681, ER-1685 and ER-1711 products, which respectively contain 32%, 36% and 40% by weight of zirconia on average.
The above products contain zirconia which is referred to as xe2x80x9cfreexe2x80x9d or xe2x80x9cprimaryxe2x80x9d zirconia (because it is not included in the eutectic crystals). The free zirconia crystals are small and tend to assume a spherical or nodular shape. Eutectic corundum zirconia crystals are also encountered. They have a relatively isotropic shape. Free corundum crystals are often encountered in the products commercially available at present.
AZS refractories are widely used in glass furnaces, in areas in contact with the molten glass. Some new glass compositions are more corrosive with respect to the materials of which the furnace is constructed. Also, glassmakers are seeking much longer working periods (determined by the service life of the refractories). There is therefore still a need for refractories that are more resistant to corrosion by molten glass. The most sensitive area is at the flotation line. The service life of the furnace is often dependent on the wear of the materials at the flotation line. Also, changes in glassmaking furnace design have increased the loads imposed on the hearth of the furnace. Increased insulation of the hearth to limit the consumption of the furnace, the use of bubblers and the increasing number of electrodes passing through the hearth have led to an increase in the temperature of the hearth where it is in contact with the molten glass, which exacerbates the problems of corrosion. There is therefore a need for products having improved corrosion resistance. It is well known in the art that introducing large quantities of zirconia improves corrosion resistance. However, increasing the zirconia content increases the cost and leads to increased segregation in the product, which can reduce industrial feasibility. Also, the increased zirconia content reduces the thermal conductivity, which is disadvantageous from the point of view of the industrial corrosion rate. The rate of corrosion of a material depends on the glass/refractory interface temperature, which is in turn conditioned by the thermal conductivity of the refractory. The more insulative the refractory product and the higher its interface temperature, the greater its rate of corrosion.
There is therefore a requirement for an AZS refractory having improved corrosion resistance with no significant increase in zirconia content.
An object of the invention is to satisfy that requirement.
In-depth studies have shown that it is possible to obtain an oxidized AZS refractory with increased corrosion resistance with the same chemical composition as typically encountered nowadays, the material being characterized by a novel and improved microstructure in the active area.
The invention provides oxidized alumina-zirconia-silica (AZS) refractories containing 40 wt % to 55 wt % Al2O3, 32 wt % to 45 wt % ZrO2, 10 wt % to less than 16 wt % SiO2 and 1 wt % to 3 wt % of an alkali metal oxide selected from Na2O, K2O and mixtures thereof, having a microstructure essentially comprising alpha-alumina crystals, free zirconia crystals, eutectic crystals and an intercrystalline vitreous phase, wherein, at least in the active area, more than 20% by number of the free zirconia crystals have a dendritic shape and are interleaved with each other and with eutectic crystals and at least 40% by number of the dendritic free zirconia crystals have a dimension greater than 300 xcexcm.
A surface area of 64 mm2 of the active area of the materials preferably contains at least 200 dendritic free zirconia crystals having a dimension greater than 300 xcexcm.
The materials claimed preferably contain 45 wt % to 50 wt % Al2O3, 34 wt % to 38 wt % ZrO2, 12 wt % to 15 wt % SiO2 and 1 wt % to 3 wt % of an alkali metal oxide selected from Na2O, K2O and mixtures thereof.
For cost reasons, the alkali metal oxide is preferably Na2O.
More than 20% of the dendritic free zirconia crystals are preferably longer than 500 xcexcm.
A surface area of 64 mm2 of the active area of the materials preferably contains at least 100 dendritic free zirconia crystals having a dimension greater than 500 xcexcm.
Surprisingly, it has been shown that it is possible to obtain microstructures offering improved corrosion resistance in a reproducible and homogeneous manner in the active area for a given range of chemical composition and using the oxidizing production method. Trials have been conducted and show also that if the microstructure of the AZS materials contains free zirconia crystals at least 20% of which by number have a dendritic shape and at least 40% of which by number have a dimension greater than 300 xcexcm, corrosion resistance is improved by more than 15% relative to equivalent materials that do not satisfy this condition. Below the above thresholds, and in particular below the minimum dimension of 300 xcexcm, no significant improvement in corrosion resistance is observed, even if the total number of free zirconia crystals is large.
It has been noted that, in the case of products in accordance with the invention, almost all (at least 80%) of the free zirconia crystals more than 300 xcexcm long are dendritic free zirconia crystals.
A value of 300 xcexcm has been adopted as a critical limit for the length of the dendritic free zirconia crystals. Analysis of the microstructures of a conventional AZS product used as a reference product showed that the average length of the free zirconia crystals was less than 100 xcexcm and that the longest crystals were 250 xcexcm long. The presence of elongate crystals longer than 300 xcexcm is therefore the sign of a reinforcing. The reinforcing is significant when more than 40% by number of the dendritic free zirconia crystals satisfy this minimum length criterion.
To understand the role of these crystals in the mechanism of corrosion of AZS products it is necessary to review the various steps of the process of dissolution of the material in contact with molten glass. The phenomenon begins with the penetration of corrosive alkaline elements of the molten glass into the vitreous phase of the material. This is followed by the onset of dissolution of the alumina of the eutectic in the vitreous phase, behind the glass/refractory interface. An interface layer rich in alumina is finally created, which contains the zirconia skeleton of the material. This interface layer is very important because it protects the material. The renewal of this interface by the convection of the molten glass aggravates corrosion of the refractory. It is considered that the presence of zirconia crystals of sufficient size (greater than the dimension of the interface) and the interleaving of those crystals constitutes a reinforcement of the interface layer limiting its renewal. Reducing renewal in this way slows the process of corrosion of AZS refractories. The interleaving of the crystals, which has an important function, is possible only if the crystals concerned are of sufficiently elongate shape. Accordingly, only dendritic free zirconia crystals are taken into account.
The specified limits for the contents of Al2O3, ZrO2 and SiO2 encompass the compositions of existing conventional commercial materials. The presence of silica is necessary to guarantee industrial feasibility but must be maintained at a level less than 16% because, beyond that value, there is massive penetration in service of corrosive elements of the glass and disintegration of the material caused by strong convection currents encountered in the heaviest wear areas of modern glass-melting furnaces.
To prevent the formation of mullite and thereby encourage the formation of an intercrystalline vitreous phase rich in silica the total content of sodium oxide and/or potassium oxide must not be less than 1%. The plasticity of this amorphous phase accommodates mechanical stresses associated with cooling of the material and the change in volume associated with the allotropic transformation of the zirconia over a wide range of temperatures. These conditions ensure that the parts are industrially feasible. In contrast, to prevent problems of exudation and reduced corrosion resistance the total content of sodium oxide and/or potassium oxide must not exceed 3%.
The following description, which refers to the accompanying graph and microphotographs, clearly explains the invention and the advantages of the novel products. The examples are provided in order to illustrate the invention and are not limiting on the invention.